1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly to a zoom lens system that is suitable for use with cameras such as a hand-held video camera.
2. Background
As hand-held video cameras have recently gained increasing popularity, a demand has arisen for a suitable zoom lens that is bright, that permits a high zoom ratio, that is compact and that can be manufactured at low cost.
For practical purposes, zoom lenses suitable for use with hand-held video cameras are required to have a brightness on the order of 1.2-1.4 in terms of F number and a zoom ratio of about 10. Zoom lens systems of a four group type including a positive, a negative, a negative and a positive lens group have been proposed as systems that are capable of satisfying the above requirements (see Unexamined Published Japanese Patent Application Nos. 17042/1979, 24652/1979, 157118/1988 and 157119/1988). However, the zoom lens systems described in Unexamined Published Japanese Patent Application Nos. 17042/1979 and 24652/1979 comprise an excess amount of lens elements resulting in an inevitably high production cost. In addition, the geometry of such a system is not compact. The basic concept of the zoom lens system described in Unexamined Published Japanese Patent Application Nos. 157118/1988 and 157119/1988 is to achieve compactness and a high zoom ratio by using new types of optical devices that are capable of changing their radius of curvature. However, it would be difficult to fabricate such a new class of optical devices and the cost of manufacturing a zoom lens system utilizing such optical devices would increase.